Mechanisms of genotoxin-induced transcription and hypermutation in p53

I. VH gene transcription creates stabilized secondary structures for coordinated mutagenesis during somatic hypermutation

During the adaptive immune response, antigen challenge triggers a million-fold increase in mutation rates in the variable-region antibody genes. The frequency of mutation is causally and directly linked to transcription, which provides ssDNA and drives supercoiling that stabilizes secondary structures containing unpaired, intrinsically mutable bases. Simulation analysis of transcription in VH5 reveals a dominant 65nt secondary structure in the non-transcribed strand containing six sites of mutable ssDNA that have also been identified independently in human B cell lines and in primary mouse B cells. This dominant structure inter-converts briefly with less stable structures and is formed repeatedly during transcription, due to periodic pauses and backtracking. In effect, this creates a stable yet dynamic "mutability platform" consisting of ever-changing patterns of unpaired bases that are simultaneously exposed and therefore able to coordinate mutagenesis. Such a complex of secondary structures may be the source of ssDNA for enzyme-based diversification, which ultimately results in high affinity antibodies.

II. Correlations between secondary structure stability and mutation frequency during somatic hypermutation

The role of secondary structures and base mutability at different levels of transcription and supercoiling is analyzed in variable region antibody genes VH5, VH94 and VH186.2. The data are consistent with a model of somatic hypermutation in which increasing levels of transcription and secondary structure stability correlate with the initial formation of successive mutable sites. Encoded differences exist in stem length and the number of GC pairs at low versus high levels of transcription in CDRs. These circumstances simplify the complexities of coordinating mutagenesis by confining this process to each mutable site successively, as they form in response to increasing levels of transcription during affinity maturation.

Identification of somatic hypermutations in the TP53 gene in B-cell chronic lymphocytic leukemia

Abnormalities of the TP53 gene are associated with a particularly severe prognosis in patients with B-cell chronic lymphocytic leukemia (B-CLL). This tumor-suppressor is mostly inactivated by the deletion of one and point mutation of the other allele and has not been previously shown to be hypermutated in B-CLL. We identified two patients whose lymphocytes showed repeatedly an extensive proportion of TP53 mutated cells by FASAY analysis (the yeast functional assay) and harbored various TP53 mutations, mostly single-base substitutions, in individual cells. The mutation targeting exhibited characteristic traits of the somatic hypermutation process. In the first patient (harboring the unmutated IgVH locus) a significant bias to point mutations at CG pairs (21/25; P=0.009), their remarkable preference for the RGYW/WRCY motives (28%) and the highest expression of the activation-induced cytidine deaminase (AID) mRNA among the 34 tested B-CLL samples. In the second patient no CG bias was observed but the targeting of point mutations into the RGYW/WRCY motives was even more prominent here (7/16; 44%). Moreover, six out of eight point mutations affecting AT pairs were localized in the WA/TW motives, which are also characteristic for the somatic hypermutations. This patient, who was IgVH-mutated, already did not express any significant amount of the AID transcript. Our findings add a new aspect to the mosaic of the p53 mutability in B-CLL.